In hospital and hotel-motel applications for television receivers, it is frequently desired to provide audio reception on one or more unused signal channels of a closed circuit system. Also, it is desirable to present a blanked raster or a blanked viewing screen for the channel selected to provide the audio reception. Thus, audio information, usually in the form of a radio transmission signal, is conveyed by an unused signal channel in a television receiver which also presents a blanked viewing screen to the observer.
In the prior art is has been customary to provide mechanical switch of the rotary type attached to the back of the signal channel tuner cluster of the television receiver. The switch included a positional location for each signal channel of the tuner and could be positionally adjusted to kill or render inoperative the scanning raster for any particular signal channel. More specifically, the switch usually opened the cathode circuit of the horizontal output tube for any particular programmed signal channel. In this manner, the selected signal channel provided audio reception which was not deleteriously affected by a scanning raster on the viewing screen of the television receiver.
Although the above-described apparatus has been and still is employed in numerous applications, it has been found that such systems do leave something to be desired. More specifically, it has been found that the above-mentioned mechanical switching apparatus is relatively expensive to provide. Also, the mechanical switch requires extensive electrical and mechanical alterations to a standard television receiver. Moreover, the mechanical-type switch undesirably requires programming for each signal channel whereat raster blanking is desired.